Ceramic catalysts may be employed to induce/increase a range of different chemical reactions such as synthesis of organic and inorganic compounds, decomposition of oxides, oxidation of compounds etc. One example of use of ceramic catalysts are in de-nitrification of exhaust/flue gases. Nitrogen oxides are unwanted bi-products often formed in combustion processes and other chemical reactions involving nitrogen and oxygen at high temperatures. Many of these oxides have detrimental effects on the environment. For example, N2O is a very strong greenhouse gas, and the Kyoto Protocol calls for strong reductions in the emissions of the gas. NO and NO2 are strong acidic oxides forming nitric acid when reacting with water, and is a major source for acidic rain. Nitrogen oxides may be catalytically reduced to elementary nitrogen and oxygen by use of specific classes of ceramic catalysts.
The catalytic active compound in ceramic catalysts may be the ceramic compound themselves, or the catalytic activity may be provided by compounds added to the ceramics, either in form of a composite in the ceramic matrix or of a coating on the ceramic solids.
Ceramic catalysts may advantageously have a plurality of through-going channels in order to increase the surface area of the catalysts. These channels are usually oriented in parallel inside each solid catalyst, and the catalyst body is usually given a cylindrical shape with non-circular cross section, that is, the catalyst object is an elongated object with an n-fold rotational symmetry along its centre axis. The catalysts may also be given a complementary form (not necessarily with n-fold rotational symmetry) such that they may be adjoined to form a planar inter-locking structure with the internal channels oriented normal to the surface plane of the structure in order to force passing fluids to flow through the channels. The physical dimensions of the planar inter-locked structure may be designed to cover the entire cross section of a reactor in order to force all fluids flowing through the reactor to pass through the channels in the catalyst.
Solid ceramic catalysts with the form of cylinders with or without circular cross sections and which incorporate a plurality of through-going channels in parallel with the centre axis are often denoted monoliths in the literature and industry. These structures are usually either coated with a catalytically active layer or are actually produced from the catalyst itself (S. Irandoust and B. Andersson, Catal. Rev.-Sci. Eng, 30, 341-392 (1988), “Monolithic catalysts for non-automobile applications”, and A, Cybulski and J. A. Moulin, Catal. Rev.-Sci. Eng, 36, 179-270, (1994), “Monoliths in heterogeneous catalysis”). Monoliths are usually given a complementary shape and orderly placed side by side with their channels directed along the flow direction in the reactor such that the monoliths completely cover the cross-sectional area of the reactor. Thus the gas flowing in the reactor is made to enter the channels and passing through them.
Relatively small (as compared to monoliths) pellet-like ceramic catalysts with internal channels are often denoted “miniliths” in the literature and industry. These catalysts may be given any shape and are usually placed in random order in a layer covering the cross-section of the reactors. The direction of the internal channels in the catalysts will thus become stochastic, and the main catalytic effect is obtained by forcing the reactant fluids to flow through the layer of miniliths in-between the miniliths. Only a fraction of the reactant flow will enter and pass through the internal channels.
The catalytic activity of a solid catalyst is usually a function of amount of catalyst material and the contact area of the solid. As a general rule, the catalytic activity is controlled by the amount (mass) of the catalytic components when dealing with reactions with relatively slow chemical reaction rates, and by surface area when dealing with relatively rapid chemical reaction rates. The fast reactions occurs predominantly on the surface of the catalyst since the reactants becomes reacted before they are able to diffuse into the bulk phase of the catalyst, and is thus a surface controlled process. The slower chemical reactions allow time to involve the bulk phase of the catalysts and thus become more bulk phase (i.e. mass) controlled process.
The ratio of amount of catalyst mass and surface area is a function of number and dimensions of the through-going channels, i.e. the voidage fraction becomes a measure of the catalytic activity of the ceramic catalyst. Catalysts intended for mass controlled reactions should have voidage fractions in the range from about 30% to 50%, while catalysts intended for surface controlled reactions should have voidage fractions from about 60 to 90%.